Abstract: Migration as a Process to Create Abnormally High Gas Contents in the Ferron Sandstone Coal Beds, Central Utah

TABET, DAVID E., Utah Geological Survey

Introduction

High volatile C and B bituminous rank coals of the Ferron Sandstone, in the central Utah area from Price to Castle Dale, are prolific producers of methane. According to River Gas Corporation and Texaco Exploration and Production, major developers of the Ferron coalbed gas play, coals of this rank typically contain 80 cubic feet of gas per ton. However, gas contents measured from cores of these coals are reported to average 400 cubic feet of gas per ton indicating super-saturated conditions. This study indicates that the abnormally high gas content of the Ferron coals in Price to Castle Dale area is the result of southward migration of gas from deeply buried, more mature Ferron coal in the Uinta Basin.

Ferron Sandstone Maturity and Burial History

Early Utah Geological Survey (UGS) research on the origin of coalbed gas (CBG) in the Ferron Sandstone looked for a western source of the gas, but found that the western margin of the Ferron coal swamp had never been buried deep enough to generate thermogenic methane. The UGS's examination of Ferron coal samples shows that mean maximum vitrinite reflectance increases only slightly from about 0.5 percent on the eastern, near-surface part of the play, to a maximum of 0.71 percent at a depth of 5,622 feet under the Wasatch Plateau to the west. A cross-plot of vitrinite reflectance versus sample depth showed that significant gas generation from Ferron coals requires burial depths greater than 9,300 feet. As presently exhumed, most Ferron coals under the Wasatch Plateau are currently buried less than 7,500 feet deep, and the historic maximum burial depth is estimated to be less than 9,300 feet, except at the north end of the Plateau. The Ferron trend extends northward from the Wasatch Plateau into the Uinta Basin where the coals are currently buried in excess of 9,300 feet.

Time-temperature plots of burial history for three wells following the Ferron Sandstone coal trend northward into the Uinta Basin show increasing maturity of the Ferron Sandstone to the north. Vitrinite reflectance data from the U.S. Geological Survey (USGS) for the Cretaceous Mesaverde Group coals of the Uinta Basin, which should mirror that of the deeper Ferron Sandstone coals because of similar burial histories, suggest that the Ferron coals, at depth, achieved the stage of significant thermogenic CBG generation about 15 miles north of Price, the locus of current CBG production.

Coalbed Gas Composition

Low-rank Ferron coals in the early thermogenic stage of maturation should not have super-saturated gas contents like those reported by River Gas and Texaco. High gas content could have come from biogenic sources, but USGS composition and isotopic analyses of the Ferron CBG indicate the gas was thermogenically derived from coal. As expected, the produced coalbed gas is very dry, with methane comprising 99 percent of the hydrocarbons. Although the carbon isotopic composition of migrated gases are generally assumed to remain constant in conventional reservoirs, the effect of migration on isotopic composition in coal reservoirs is less well understood. Coals from the San Juan Basin also have carbon isotopic compositions lighter than expected. Such an isotopic composition could result from the migration of thermogenic coalbed gas where lighter isotopes migrated faster than heavier ones. Hence, gas migrating out of the Uinta Basin into the Ferron coals of the Price-Castle Dale area of central Utah would be expected to be both dry and isotopically light.

Ferron Sandstone Hydrology

Another piece of evidence supporting the southward migration of methane from deep Ferron coals in the Uinta Basin comes from a new UGS potentiometric map for the Ferron Sandstone of central Utah. This map shows different hydrologic gradients for the northern and southern parts of the Ferron Sandstone trend. At the southern end of the Ferron trend, regional groundwater flow is north and east toward the town of Castle Dale. Conversely, the northern end of the Ferron trend has a hydrologic gradient with regional flow southward out of the Uinta Basin toward Castle Dale. Assuming that this hydrologic pattern has existed since Ferron burial, then water flowing out of the Uinta Basin could have carried methane into the shallower coals in the Price-Castle Dale area. The potentiometric surface for the Ferron aquifer lies above the top of the Ferron Sandstone indicating that the aquifer is artesian and the Ferron is slightly over-pressured.

Conclusions

Several lines of geologic information point to deeper Ferron coals in the Uinta Basin as the source of the gas presently produced in the Ferron CBG play of central Utah. Analysis of burial history and maturity data indicate that burial depth and maturity increase to the north and reach the stage of significant thermogenic gas production in the Uinta Basin. The dry nature and light isotopic composition of the Ferron CBG indicates the gas could have migrated from a thermogenic coal source. Finally, regional hydrologic trends for the Ferron Sandstone support southward water flow and gas migration out of the Uinta Basin toward the Price-Castle Dale area.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah